docs: def _get_linewidth(tree, linewidth, diameter_scale):
      '''calculate the desired linewidth based on tree contents
  
      If diameter_scale exists, it is used to scale the diameter of each of the segments
      in the tree
      If diameter_scale is None, the linewidth is used.
      '''
      if diameter_scale is not None and tree:
          linewidth = [2 * segment_radius(s) * diameter_scale
                       for s in iter_segments(tree)]
      return linewidth